Search

Multiphoton absorption and multiple excitation can lead to the formation of highly electronically excited states of molecules. We have been applying these excitation methods to explore specific photochemical reactions, which are rather difficult to attain by normal one-photon absorption processes. In the present review, we will introduce several examples of these photochemical responses specific to highly excited state in the condensed phase, such as two-photon-gated cycloreversion, one-color control of both reactions in photochromic systems and rapid capture of an electron ejected from the higher excited state leading to rapid generation of charge-separated states at the high energy level with a lifetime much longer than microseconds.

We have investigated three-dimensional distribution and diffusion behaviors of single guest dyes in 1-µm thick films of poly(2-hydroxyethyl acrylate) (PHEA) by using astigmatism imaging method. Perylene diimide derivative (BP-PDI) in the PHEA films localized along the Z-axis at ca. Z = 600-700 nm distant from the interface (Z = 0) between PHEA and glass substrate. This Z-localization was not observed in different polymer films of poly(methyl methacrylate) (PMMA), poly(methyl acrylate) (PMA), and polystyrene (PSt). Because the glass transition temperature of the PHEA is lower than the room temperature, BP-PDI in the PHEA films exhibited Brownian motion, normal diffusion on the XY plane and confined motion along the Z-direction. For elucidating the mechanism of the peculiar localization of the guest dyes along film thickness in the PHEA films, we measured diffusion behaviors of different dyes, R6G and Atto 488, in 1-µm thick PHEA films, obtaining result that the Z-distributions of the dyes were overall similar to that of BP-PDI. The result indicates that the Z-localization of the guest dyes should be ascribed not to the interaction between glass surface and guest dye but mainly to the Z-dependent property of the PHEA film. Indeed, the lateral diffusion coefficients of the guest dyes depended on their Z-positions.

Using one-color fluorescence-switching of diarylethene derivatives (DAEs), thousands of single-molecule translational trajectories in a blended polymer solid were measured with nanometric accuracy.

We have demonstrated in the present report that dielectric microparticles exhibited orbital rotation in the light field of non-coaxially configured two counter-propagating laser beams both in numerical simulations and experiments. A series of computational simulations indicated that when irradiated with two non-coaxially counter-propagating parallel laser beams with same intensity distributions in the absence of thermal (Brownian) motion, a microparticle did not exhibit orbital rotation due to the symmetry of the optical field. However, the computations predicted that a microparticle exhibited one directional orbital rotation in the presence of thermal motion because of the symmetry breaking of the optical force acting on the particle. This spontaneous orbital rotation was experimentally demonstrated for 1-µm dielectric particles in water at room temperature.

Photomechanical molecular crystals are promising materials as photon-powered artificial actuators. To design and control the photomechanical responses, the spatiotemporal distribution of photoproducts in crystals could be a key factor in addition to molecular structures, molecular packings, illumination conditions, crystal morphology, crystal size, and so on. In this study, we have found that single crystals of 2,5-distyrylpyrazine show a smooth single-crystal-to-single-crystal photomechanical expansion, and the photochemical reaction propagates from the edge to the center of the single crystal. Moreover, we have performed a numerical simulation to reproduce the experimental results and revealed that both the cooperativity effect and the surface effect in the crystal are essential for the edge-to-center propagation of the photochemical reaction. Our results would provide a framework for analyzing the photochemical reaction dynamics in photoresponsive single crystals and a benchmark for future studies of photomechanical molecular crystals.

Multicarrier dynamics in heterostructured ZnS-AgInS

Despite the excellent optoelectronic properties of halide perovskites, the ionic and electronic defects adversely affect the stability and durability of perovskites and their devices. These defects, intrinsic or produced by environmental factors such as oxygen, moisture, or light, not only cause chemical reactions that disintegrate the structure and properties of perovskites but also induce undesired photoluminescence blinking to perovskite quantum dots and nanocrystals. Blinking is also caused by the nonradiative Auger processes in the photocharged quantum dots or nanocrystals. Herein, we find real-time suppression of halide vacancy-assisted nonradiative exciton recombination and photoluminescence blinking in MAPbBr3 and MAPbI3 perovskite quantum dots by filling the vacancies using halide precursors (MABr and MAI). Also, halide vacancy filling increases the photoluminescence quantum efficiencies and lifetimes of the quantum dots. We estimate the rates of halide vacancy-assisted nonradiative recombination at 1 × 108 s-1 for MAPbBr3 and 1.9 × 109 s-1 for MAPbI3 quantum dots. The real-time blinking suppression using the halide precursors and statistical analysis of the ON/OFF blinking time reveal that the halide vacancies contribute to the type-A blinking through charging and discharging. Conversely, the blinking of the quantum dots after halide vacancy filling is dominated by the type-B mechanism.

Photomechanical molecular crystals are promising materials as photon-powered artificial actuators. To design and control the photomechanical responses, the spatiotemporal distribution of photoproducts in crystal could be a key factor in addition to molecular structures, molecular packings, illumination conditions, crystal morphology, crystal size, and so on. In this study, we have found that single crystals of 2,5-distyrylpyrazine shows a smooth photomechanical expansion, and the photochemical reaction proceeds heterogeneously on the optical length scale by observation of the changes in color, fluorescence, and birefringence. Moreover, we have performed a numerical simulation to reproduce the experimental results and revealed that both the cooperativity effect and the surface effect in crystal are essential for the heterogeneous progress of the photochemical reaction. Our results would provide a framework for analyzing the heterogeneous reaction dynamics on optical length scale in photoresponsive molecular crystals and a benchmark for future studies of photomechanical molecular crystals.

This article reviews optical manipulation coupled with photochemical/photothermal responses of nanometer sized materials including molecular systems, polymers, and inorganic nanoparticles. After the introduction, section 2 overviews the optical trapping of nanometer sized molecular systems including early-stage studies, such as trapping of polymer chains, micelles, and molecular aggregates in solution at room temperatures. Then, the conformation control of macromolecule assemblies and gels by optical force are introduced, followed by micro-fabrications achieved by combining optical trapping and photochemical reactions. Section 3 summarizes studies on the evaluation of optical force acting on nanometric molecular systems using fluorescence correlation techniques. Approaches to control optical force by using photochemical reactions are show in section 4, where the absorption band of target materials are modified through photochromic reactions, leading to micromechanical motion of small particles synchronizing with the photochemical reactions. Section 5 overviews photothermal effect in optical manipulation such as natural convection, Marangoni convection and thermophoresis, and applications of the thermal effects to develop new methods of micromanipulation achieved by combining optical force and photothermal responses.

The design of an object transportation system exploiting the bending behavior of surface-assembled diarylethene crystals is reported. A photoactuated smart surface based on this system can transport polystyrene beads to a desired area depending on the direction of the incident light. Two main challenges were addressed to accomplish directional motion along a surface: first, the preparation of crystals whose bending behavior depends on the direction of incident light; second, the preparation of a film on which these photochromic crystal plates are aligned. Nuclei generation and nuclear growth engineering were achieved by using a roughness-controlled dotted microstructured substrate. This system demonstrates how to achieve a mechanical function as shown by remote-controlled motion along a surface.

Organic luminescent materials have attracted much attention due to potential application to organic optoelectronics. Search for new molecules with high fluorescence quantum yield in solid states is one of the most important tasks in the research field of the organic luminescent materials. Here, we newly synthesized diarylethenes, 1,2-bis(3-methyl-5-(4-alkylphenyl)-2-thienyl)perfluorocyclopentenes having methyl, ethyl, n-propyl, and n-butyl substituents at the p-position of phenyl rings, and investigated their fluorescence properties in the solid states. The diarylethenes in the crystalline phase exhibited strong fluorescence with relatively high fluorescence quantum yields (Φf) of 0.12–0.20 compared with those in the amorphous phase (Φf = 0.064–0.069), which indicates that the diarylethenes have crystallization-induced emission (CIE) characteristics. The diarylethene having methyl group as the alkyl chain has the highest Φf value among the diarylethenes that we synthesized in this work. We found that the amorphous solid of the diarylethene having methyl group was crystallized after mechanical scratching followed by heating at 90 °C. We successfully demonstrated reversible fluorescence recording based on CIE characteristics and mechanical scratching and heating induced crystallization.

Optical tweezers are powerful tools to trap, transport, and analyse individual nano-objects at dilute concentrations. However, it is still challenging to manipulate isolated single nano-objects in dense target environments with various kinds of materials, such as in living cells and mixtures of nanocolloids. In the present work, we have succeeded in the selective trapping of a few gold nanoshells with specific sizes and sweeping others out completely, only by irradiating the dense colloidal suspension of gold nanoshells with a focused near infrared continuous-wave (CW) laser. This was achieved by an interplay between optical gradient- and thermophoretic forces: while the gradient force traps the targets at the focal spot, the thermophoretic force pushes others out from the focal spot. The distance between the trapped targets and the separated others was longer than 20 mu m, allowing us to measure plasmonic scattering spectra of the trapped targets at a single-nanoparticle level. The present method paves a way for manipulating and analysing single nano-objects in dense mixtures of targets and various kinds of materials.

In the present study, we propose a combined (hybrid) approach of optical manipulation and photochemical reactions for achieving nano-photomechanical motions with small particles. Aa photochemical reactions, we have employed P-type and T-type photochromic reactions of diarylethene (DAE) and pyranoquinazoline (PQ) derivatives, respectively. Single polymer particle containing each of the photochromic compounds was optically trapped in water with a CW visible laser. At this stage, the particle experienced mainly gradient force and was trapped at the focal point of the CW laser. The absorption force was negligible because most of the photochromic molecules in the particle were in the colorless form. UV exposure induced the photoisomerization of the photochromic molecules, resulting in the increase in the number of colored forms. As a result, the absorption force acting on the particle increased and, the position of the particle shifted towards the light propagation direction depending on experimental condition. After turning the UV light off, the particle went back to the original position. The trapped particle thus underwent reciprocal motion synchronizing with the change of photo-response due to the photochromic reactions.

In the present study, we have detected multiphoton absorption force acting on microparticles due to simultaneous twophoton absorption. A polymer microparticle including dye molecules was optically trapped with femtosecond (800 nm, 100 fs) pulse pairs with time-interval Δt in water. Under the condition that Δt > the pulse duration, the particle was trapped with absorbing small number of photons via multiphoton process. The number of photons absorbed via simultaneous two-photon absorption increased by decreasing Δt to overlap the pulse pair, leading to the micromechanical motion of the particle. By tracking the position of the trapped particle, we confirmed that the particles were pushed in the direction of light propagation and the displacement increased with decreasing Δt.

In the present chapter, we first introduce the mechanism of the multiphoton-gated photochromic reactions in diarylethene and fulgide derivatives by pulsed laser excitation, where the multiple photons lead to the specific photoresponse beyond the Kasha’s rule. In the second part, we discuss the one-color control of the activation, deactivation, and fluorescence excitation in the single-molecule detection. One-color of light takes various roles resulting in the effective single-molecule tracking via the photon-material synergetic response. In the third section, photochromic reaction is applied to the controlled translational motion of nano- to micron-sized materials under the laser trapping. Integrated photoresponses of materials, such as absorption, photochemical reaction, and the photon pressure, lead to the mesoscopic motion synchronized with the photochemical reaction.

Photocontrol of mechanical motions of small objects has attracted much attention to develop mesoscopic remote actuators. For this purpose, photoinduced morphological changes of molecules, molecular aggregates, and crystals have been extensively studied in the field of chemistry and materials science. Here, we propose direct use of momenta of light (i.e. radiation force) to control the motion of small objects, through photochromic reactions of pyranoquinazoline (PQ) derivatives. PQ is colorless in visible wavelength region while it is in closed form and undergoes photochemical ring-opening reactions to form colored isomers upon UV light irradiation; the open-ring isomers return to the colorless closed isomers mainly through the thermal back reaction. In the experiment, individual polymer microparticles with diameters of 7 μm incorporating PQ were trapped by optical tweezers. When the trapped microparticle was irradiated with UV light, the microparticle was pushed along the axis of light propagation about a...

Environment-sensitive fluorophores have been used as molecular probes to acquire physical and chemical information in microstructures. Here, we report on solvatochromism of a fluorescent diarylethene, which is switched by photoirradiation. A turn-on mode photoswitchable fluorescent diarylethene derivative, 1,2-bis(2-ethyl-6-(5-methylthiophen-2-yl)-1-benzothiophen-1,1-dioxide-3-yl)perfluorocyclopentene (1a), underwent a cyclization reaction upon irradiation with ultraviolet (UV) light to form fluorescent closed-ring isomer 1b. 1b dramatically changed the fluorescent color in response to solvent polarity, such as green in n-hexane, yellow in tetrachloromethane (CCl4), orange in dichloromethane (CH2Cl2), and red in dimethyl sulfoxide (DMSO), and showed high fluorescence quantum yields of 0.6–0.8 in these solvents. The fluorescent solvatochromism is ascribed to the intramolecular charge-transfer (ICT) character of 1b, which consists of electron-donating thiophene rings at both ends and central electron-accepting benzothiophene 1,1-dioxide groups. The analysis based on the Mataga-Lippert plot revealed that the difference in the dipole moments between the excited and ground states of 1b is as large as 12 D. Such a solvatochromic fluorophore with photoswitching ability can find potential applications in super-resolution fluorescence imaging of microscopic polarity in biological cells and materials.

Photoswitchable turn-on mode fluorescent molecules have been so far successfully used in super-resolution fluorescence microscopies. Here, we report on fluorescence photoswitching of 1,2-bis(2-ethyl-6-phenyl-1-benzothiophene-1,1-dioxide-3-yl)perfluorocyclopentene (1) by irradiation with single-wavelength visible (420 nm < λ < 470 nm) light, the wavelength of which is longer than the 0-0 transition of open-ring isomer 1a, without UV light excitation. By absorbing very weak hot bands or Urbach tails 1a underwent a cyclization reaction to produce fluorescent closed-ring isomer 1b. Both cyclization and cycloreversion reactions of 1 took place with the visible light in the far off-resonance region of the absorption edge. Based on numerical simulations of the formation process of 1b from 1a by irradiation with 450 nm light, weak absorption coefficients at 450 nm in n-hexane and CCl4 were estimated to be 0.084 and 0.19 M-1 cm-1, respectively. The reversible fluorescence photoswitching with the single visible light is advantageously applicable to super-resolution fluorescence imaging in biological systems.

Crystals consisting of the closed-ring form of 1,2-bis(3-methyl-5-phenyl-2-thienyl)perfluorocyclopentene underwent a photochemical ring-opening reaction accompanying crystal fragmentation upon irradiation with visible light. The open-ring form crystal produced by the ring-opening reaction exhibited green fluorescence, whereas open-ring form crystals produced by recrystallization exhibit orange or yellow fluorescence depending on the polymorphic forms. The fluorescence quantum yield of the photogenerated open-ring form crystal was larger than that in n-hexane. The open-ring form exhibits different fluorescence colors depending on the intermolecular interaction in different states.

Turn-on mode fluorescence switching molecules are applicable to super-resolution fluorescence microscopy, such as PALM (photoactivation localization microscopy), STORM (stochastic optical reconstruction microscopy) or RESOLFT (reversible saturable (switchable) optical linear fluorescence transitions) microscopy. Here we report on a molecular design strategy to control the cycloreversion quantum yield of the turn-on mode fluorescent diarylethenes, 1,2-bis(2-ethyl-6-aryl-1-benzothiophene-1,1-dioxide-3-yl)perfluorocyclopentenes. Diarylethene derivatives having ortho-substituted phenyl rings at 6- and 6′-positions of 1,2-bis(2-ethyl-1-benzothiophene-1,1-dioxide-3-yl)perfluorocyclopentene were synthesized and their photophysical and photochemical properties were evaluated. The ortho-substitution of the phenyl rings with fluorine or methyl groups decreases the extension of π-conjugation length, resulting in a small blue shift of the absorption spectrum and a substantial increase in the cycloreversion quantum yield from 10−4 to 10−2 without significant influence on the fluorescence quantum yield. Water-soluble derivatives having sodium sulfonate groups were also synthesized. These derivatives fulfill requirements necessary for the application to RESOLFT super-resolution fluorescence microscopy.

We have investigated the excitation wavelength and intensity dependencies of fabrication of silver nanoparticles (Ag NPs) on a glass substrate through the laser-induced photoreduction of silver ions in aqueous solution. It was found that the nucleation of Ag NPs could occur not in an aqueous solution of silver nitrate but at a water-glass interface without any reducing reagents. The nucleation could be triggered by photoexcitation at visible to Near-IR wavelength regions where the silver nitrate solution has no absorption band. From the excitation wavelength dependence using five CW lasers at 325, 442, 532, 690, and 1064 nm, it has been presumed that one-photon absorption of unique surface chemical species such as silver oxide (Ag 2 O) on the glass substrate is responsible for the reaction leading to the nucleation. Numerical simulation of a single Ag NP and a Ag@Ag 2 O core-shell NP on the basis of Mie theory revealed the photoproducts consists of Ag, but not Ag 2 O.

Luminescent solid-state materials with high fluorescence quantum yield have attracted much attention in application to organic optoelectronics. In the course of study of photochromic diarylethene crystals, we found that two polymorphic forms of a diarylethene are obtained by recrystallization from acetone and n-hexane solutions. Surprisingly, both polymorphic crystals exhibited strong emission whose fluorescence quantum yield (>0.5) is much higher than that in n-hexane solution (0.017). Furthermore, the thermodynamic solid-state polymorphic phase transition was found and the process was directly visualized by a fluorescence color change.

When plasmonic nanoparticles are placed on a highly refractive semiconductor substrate, we can expect three different effects: (i) resonance mode splitting, (ii) asymmetric light scattering based on the split modes, and (iii) site-selective nanoetching due to plasmon-induced charge separation (PICS) at the nanoparticle–semiconductor interface. Here, we develop novel photofunctional materials by taking advantage of those three effects. More specifically, we control the asymmetric scattering of Ag nanocubes on TiO2 by PICS, so as to develop materials for photodrawing of one-way visible translucent images and multicolor scattering images. The one-way visible translucent images, which are translucent scattering images visible only from the back side, are drawn by anaerobic bottom-selective etching of the Ag nanocubes. For drawing the multicolor scattering images, a scattering color of Ag nanocubes is changed from yellow to green by the anaerobic bottom-selective etching and from yellow to red by aerobic nonse...

A diarylethene–fluorene dyad connected by an ester bond (DE-FL) and random and alternative copolymers with diarylethene and fluorene moieties in their side chains (poly(DE1x-co-FL1y) and poly(DE1-alt-FL2)) were synthesized, and their fluorescence on/off switching properties were investigated. All compounds exhibited reversible photochromism and fluorescence on/off switching in a solution upon alternating irradiation with ultraviolet and visible light. Although the fluorescence intensity of DE-FL linearly decreased with increasing photocyclization conversion of the diarylethene, the fluorescence intensity of poly(DE1x-co-FL1y) at the same molar fraction of diarylethene/fluorene significantly decreased with photocyclization conversion. Moreover, the fluorescence intensity of the polymers in the photostationary state was much lower than that of DE-FL. These results indicate that the fluorescence on/off switching properties of the polymers were improved by introducing diarylethene and fluorene moieties into t...

We demonstrated one-colour control of activation, excitation and deactivation of a fluorescent diarylethene derivative by using a 532 nm CW laser at a weak output power. This one-colour control method was applied to single-molecule tracking in polymer films over a total duration of a few hours at room temperature and PALM for the nanostructures of polymer systems.

Gold nanoparticles (Au NPs) efficiently convert incident light into heat under the resonant conditions of localized surface plasmon. Controlling mass transfer through plasmonic heating of Au NPs has potential applications such as manipulation and fabrication within a small space. Here, we describe the formation of stationary microbubbles and subsequent fluid convection induced by CW laser heating of Au NPs in water. Stationary bubbles of about 1–20 μm in diameter were produced by irradiating individual Au NPs with a CW laser. Spatial profiles and velocity distribution of fluid convection around the microbubbles were visualized by the wide-field fluorescence imaging of tracer nanospheres. To evaluate the bubble-induced convection, numerical simulations were performed on the basis of general heat diffusion and Navier–Stokes equations. A comparison between the experimental and computational results revealed that a temperature derivative of surface tension at the bubble surface is a key factor to control the fluid convection. Temperature differences of a few Kelvin at the bubble surface resulted in convective velocities ranging from 102 to 103 μm s−1. The convective velocity gradually increased with increasing bubble diameter. This article covers both natural and Marangoni convection induced by plasmonic heating of Au NPs.

Photocyclization reaction dynamics of an inverse type diarylethene derivative was investigated in alkane solutions by means of ultrafast laser spectroscopies. Femtosecond transient absorption spectroscopy showed that the Franck–Condon state formed by photoexcitation is geometrically relaxed to a transient species within 100 fs and subsequently the cyclization process takes place with a time constant of 36 ps. This time constant is much longer than those in normal type derivatives. Steady-state and time-resolved fluorescence measurements with the aid of quantum chemical calculations revealed that there exist three kinds of conformers, one parallel and two anti-parallel forms, in the ground state. One of the anti-parallel conformers undergoes the cyclization reaction, while the other two conformers are nonreactive species and their major relaxation processes are radiative decay and intersystem crossing into the triplet states. The triplet states thus formed no longer undergo the cyclization reaction in the late time region.

We synthesized fluorene derivatives having phenylthiophene (FPT) or benzothiophene (FBT), and their S,S-dioxidized compounds (FPTO2, FPTO4, FBTO2 and FBTO4), which are prepared by oxidation of the thiophene rings in FPT and FBT with m-chloroperoxybenzoic acid. FPT and FBT exhibited similar optical properties for absorption maximum wavelength, fluorescence maximum wavelength and fluorescence quantum yield. However, the absorption and fluorescence spectra of FPTO2 were largely shifted toward a longer wavelength in comparison with those of FPT, and their fluorescence quantum yields dramatically decreased with oxidation. In contrast, the absorption and fluorescence spectra and the fluorescence quantum yields of FBTO2 and FBTO4 were similar to those of FBT. Moreover, FPTO2 and FBTO2 showed strong solvatofluorochromism. Such solvent dependent properties are ascribed to the charge transfer character of the molecule.

Femtosecond transient absorption (TA) and picosecond time-resolved fluorescence (TRF) spectroscopies were applied to the charge separation (CS) dynamics of 10-cyano-9,9′-bianthryl (CBA) in a normal polar solvent, acetonitrile (Acn), and in a highly viscous room temperature ionic liquid (IL), N,N-diethyl-N-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4). The primary CS took place within the ultrafast sub-100 fs time range in both solvents, which was completely independent of diffusive solvation. Subsequent viscosity-dependent spectral evolution was observed by the TA measurement in the picosecond range which was ascribed to the structural relaxation. A red shift of the TRF spectrum in the picosecond to nanosecond range was observed in DemeBF4 which was due to the diffusive solvation in the CS state. Interestingly, integrated fluorescence intensity decayed more rapidly than TA in the IL, while they decayed simultaneously in Acn. It was concluded that diffusive solvation decreases the radiative t...

Asymmetric diarylethene derivatives having benzophosphole and benzothiophene groups at the aryl moieties were synthesized and their photochromic properties were examined. Three diarylethene derivatives, 1 having benzophosphole and benzothiophene groups, 2 having benzophosphole P-oxide and benzothiophene groups, and 3 having benzophosphole P-oxide and benzothiophene S,S-dioxide groups, were synthesized by appropriately controlled oxidation of benzophosphole and benzothiophene groups. Diarylethenes 1–3 underwent photochromic reactions in acetonitrile as well as in the powder solid state upon alternate irradiation with ultraviolet (UV) and visible light. The closed-ring isomer of diarylethene 3 emitted fluorescence with a quantum yield of 0.05 in acetonitrile solution. The fluorescence quantum yield dramatically increased up to 0.55 in the solid state.

The photochromic and fluorescence properties of two diarylethene derivatives having benzo[b]thiophene S,S-dioxide groups were studied in the single-crystalline phase. The derivatives showed reversible photochromic reactions and turn-on mode photoswitching of fluorescence in solution as well as in the single-crystalline phase. Upon irradiation with UV light, the open-ring isomers in the crystals underwent cyclization reactions to produce fluorescent closed-ring isomers. The UV-irradiated crystals emitted green or yellow-green fluorescence and exhibited dichroism of their absorption and fluorescence spectra under linearly polarized light. Upon irradiation with visible light, the closed-ring isomers reverted to the open-ring isomers and the fluorescence of the crystals disappeared. The switching cycles could be repeated more than 50 times without detectable deterioration. Area-selective switching of the fluorescence in the single crystal was demonstrated by patterned light irradiation, suggesting potential applications of the photoswitchable fluorescent molecular crystals in optical memory and display devices.

Reprecipitation of pyrene (Py) in the glassy solution of methylcyclohexane and isopentane at 77 K was observed by the repetitive irradiation of nanosecond (ns) laser pulses at 355 nm. The dynamics and mechanism of this reprecipitation were investigated by means of time-resolved fluorescence and absorption spectroscopies. Although only the fluorescence of the Py monomer was observed before the ns laser irradiation, fluorescence of the excimer was observed during the initial one-shot laser irradiation. From the time-resolved fluorescence measurements, it was revealed that the appearance of the excimer was due to the transient melting of the glassy medium by the local temperature increase around Py, which was induced by the iterative reabsorption of the laser light by the S1 state. The time period of melting for allowing the translational diffusion of Py was limited in the time region ≤ ca. 10 ns. With an increase in laser exposure, the fluorescence intensity of the excimer increased concomitantly with the appearance and increase of the amount of Py dimer, which was also confirmed by steady-state absorption spectroscopy. Time-resolved fluorescence spectrum recorded by only the one-shot laser exposure did not show dimer emission. This suggested that the formation of the dimer was through the excimer produced by transient melting; its dissociation into monomers was prohibited in the highly viscous environment. Upon further increase in laser exposure (several 1000 shots), solidified Py was observed due to crystal formation/aggregation with the dimers as the nucleation species.

Not only the energy but also the momentum of photons transfers to material via photoabsorption; this momentum transfer, known as radiation pressure, can induce motions of small particles. It can therefore be expected to induce mechanical motions of mesoscopic materials synchronized with the reversible change of their absorption coefficient by external stimuli. We demonstrated quantitative photomechanical motions in mesoscopic regions by combining optical tweezer and photochromic reactions of diarylethene (DAE). A microparticle including DAE was optically trapped with 532 nm laser and the absorption band of the DAE was photoswitched with UV laser, resulting in the modulation of the radiation force through the change in the complex dielectric constant of the particle. In this process, mesoscopic mechanical motions were successfully induced by the photochromic reaction. The present approach is potentially applicable in a wide variety of nano/micromechanical devices and also paves the way for monitoring the absorption of photons by molecules via photomechanical response.

We fabricated semiconductor cadmium selenide (CdSe) quantum dots via the pulsed laser ablation in the superfluid helium. The fabricated quantum dots showed blue-shifted fluorescence due to the strong quantum confinement effect. The fluorescence blinking phenomena were also observed indicating the single photon emission process. Our proposed scheme is a simple, robust, and reliable method to fabricate quantum dots and to introduce the highly fluorescence nanoparticles into superfluid helium appropriate for resonant optical manipulation and nano-tracers for liquid helium visualization.

Single- and double-component nanoparticles consisting of two types of diarylethenes, 1,2-bis(3-methyl-5-phenyl-2-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (2a), were fabricated by a reprecipitation method. Nanoparticles consisting of 1a exhibited orange or green fluorescence depending on the fabrication condition and did not undergo any photocyclization reaction. On the other hand, nanoparticles consisting of 2a underwent photoreversible photochromic reactions upon alternating irradiation with ultraviolet and visible light. Nanoparticles consisting of 1a and 2a exhibited fluorescence on/off switching with rapid switching speed and high on/off contrast, accompanying the photochromic reactions of 2a. The dependence of fluorescence on/off switching properties on Forster distance and molar fraction was observed and quantitatively evaluated by a simplified model.

A new fluorescent photochromic diarylethene that can be activated by irradiation with 405 nm light was synthesized. The turn-on mode switching of fluorescence with visible light is favorable for application to biological systems. The fluorescence quantum yield of the photogenerated closed-ring isomer was as high as 0.8 in less or medium polar solvents, and even in polar acetonitrile the yield was higher than 0.6.

3D single-molecule tracking revealed that the translational diffusion of guest dyes in poly(2-hydroxyethyl acrylate) thin films on glass substrates was confined in a horizontal layer at a distance longer than 300-700 nm from the surface of the substrate. This peculiar long-range effect suggests that the interaction between the host polymer and the interface could affect the properties of polymers at a much longer distance than conventionally estimated.

Understanding the polymorph phenomenon for organic crystals is essential for the development of organic solid materials. Here, the fluorescence study of the evaporative crystallization of 1,3-dipyrrol-2-yl-1,3-propanedione boron difluoride complex (1), which has three polymorphs showing different emission profiles, is reported. The droplet of 1 in 1,2-dichloroethane showed blue emission just after dropping. Solids with bluish-green emission were observed. As time elapsed, a solid with red or orange emission was observed around the droplet. Time evolution of the fluorescence spectra, observed for the first time, implied that the molten state of 1 was observed by emission of an intermediate, even at ambient temperature. These findings suggested that the liquid-like cluster incidentally forms an ordered array as the crystallites nucleate. The liquid-like cluster can be considered as the "crucible" in the nucleation of polymorphs.

Single‐molecule fluorescence resonance energy transfer (smFRET) is a powerful tool to investigate the dynamics of biomolecular events in real time. However, it requires two fluorophores and can be applied only to dynamics that accompany large changes in distance between the molecules. Herein, we introduce a method for kinetic analysis based on control of fluorescence blinking (KACB), a general approach to investigate the dynamics of biomolecules by using a single fluorophore. By controlling the kinetics of the redox reaction the blinking kinetics or pattern can be controlled to be affected by microenvironmental changes around a fluorophore (rKACB), thereby enabling real‐time single‐molecule measurement of the structure‐changing dynamics of nucleic acids.

Remote acceleration of a molecular recognition will open an avenue for the control of various biological functions. Here, we have developed a new principle for the rapid macroscopic assembly based on the light-induced molecular recognition via nanoparticles. Remarkably, as an application of this principle, we have demonstrated the submillimetre network formation triggered by light-induced hybridization of zmol-level DNA within a few minutes. This finding will be used for the rapid and highly sensitive genetic screening without fluorescent labeling.